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But multiplying by 9 is so easy! You can check you’re right if (a) the first digit of the result is 1 less than the number you started with, and (b) the digits
by michaericalribo 6y ago
But multiplying by 9 is so easy! You can check you’re right if (a) the first digit of the result is 1 less than the number you started with, and (b) the digits sum to 9
- rmetzler 6y agoThat’s how I taught my daughter.
- vaccinator 6y agobut that only works up to 10.. my method works for any number, and you could adapt it for x11... see below
- alisonkisk 6y agoLucky for us, all digits are less than 10.
- vaccinator 6y agoWhat is 23 x 9 with that trick?
- cipherzero 6y agoJust going off the trick above would t it just be: 23 x 10 - 23 = 230 - 23 = 207... I guess I’m confused what point you’re making?
- vaccinator 6y agoI was talking about the grand-parent comment's method: > You can check you’re right if (a) the first digit of the result is 1 less than the number you started with The first digit of the result is 2 ... And the number that you started with is also 2, which is not 1 less... I guess I am not smarter then a 5th grader
- michaericalribo 6y agoWow! You definitely are smarter than me, the gp, at least! You’re right that my claim isn’t right in general. Hell, not even past 10, I can now see! I’m curious, how did you find that counterexample? I kind of ‘overfitted’ my mental model to the first 10 integers. But as a statistician I’m only in the business of being approximately right...! Nice catch. 23x9=207 starts with 20 which is 3 less than 23... 24x9=216, and 21 is 3 less than 24. 11x9=99 and 9 is 2 less than 11, 12x9=108 and 10 is 2 less than 12. So 0-9 are offset by 1, 10-19 are offset by 2, 20-29 offset by 3, 30-49 offset by 4?
- vaccinator 6y agoor multiply by 10 and subtract the number that you were trying to multiply by 9. I.E.: 12 x 9 = 12 x 10 - 12 = 108